The present invention relates to a photoreceptor such as one for use in electrophotography, and a process for producing the same.
Conventional photoreceptors for use in electrophotography are made of Se optionally doped with As, Te or Sb, or are made of ZnO or CdS which is dispersed in a binder resin. These photoreceptors are defective in that they contaminate the environment, are not heat stable and have low mechanical strength. Amorphous silicon (a-Si) has recently been proposed as a material for electrophotographic photoreceptors. Amorphous silicon is characterized by the presence of "dangling bonds" formed by the breaking of Si-Si bonds, and because of this defect, amorphous silicon has many localized state levels within the energy gap. The dark resistance of the amorphous silicon is low since thermally excited carriers cause hopping conduction; on the other hand, amorphous silicon has low photoconductivity because photoexcited carriers are trapped at the localized energy levels. In order to avoid these problems, the dangling bonds may be filled up by bonding a hydrogen atom to the defective Si atom.
The amorphous hydrogenated silicon (hereunder a-Si:H) wherein the dangling bonds on Si atoms are filled with hydrogen atoms is most preferred for use in the light-sensitive layer of a photoreceptor since its resistivity changes greatly upon illumination with light in the visible or ultraviolet region.
An electrophotographic copier using an a-Si photoreceptor made of a-Si:H is depicted in FIG. 1. This copier comprises a cabinet 1 which has on its top a glass table 3 on which the original 2 is to be set. The original 2 is to be covered with a platen cover 4. Below the original table 3 is provided a horizontally moving optical scanner composed of a light source 5 and a first mirror unit 7 having a first reflective mirror 6. In order to scan the original in such a manner that the length of the optical path between the scanning point on the original and the photoreceptor around a drum 9 is held constant, a second mirror unit 20 moves at a speed equal to the travelling speed of the first mirror unit 7, and the light reflected from the table 3 passes through a lens 21 and a reflective mirror 8 to fall in a slit form on the photoreceptor drum 9. Being an image support, the drum 9 is surrounded by a corona charger 10, a developing unit 11, a transfer unit 12, a separator unit 13 and a cleaner unit 14. Paper feeder 15 delivers copy paper 18 through rollers 16 and 17, and after a toner image is transferred from the drum 9 onto this copy paper, the image is fixed in a fixing unit 19 and the paper is discharged into a tray 35. The fixing unit 19 comprises a heating roller 23 having a built-in heater 22, and a pressure roller 24; the developed image on the paper is fixed permanently by passing the paper is fixed permanently by passing the paper between the rollers 23 and 24.
The photoreceptor having an a-Si substance on the surface is sensitive to the atmosphere and moisture. The surface of the photoreceptor is also low in stability against chemical species formed as a result of corona discharge that occurs in sections 10, 12 and 13 of FIG. 1 during the electrophotographic process. Because of these phenomena, the photoreceptor having an a-Si substance on the surface has the following defects.
(1) The bulk of the a-Si layer has a fairly good structure of Si-Si bonds, but the surface of the layer has many energy levels due to defective Si-Si bonds such as discontinuous or dangling bonds. Furthermore, a-Si has a rigid structure and any defective bond that has occurred during film making will be permanently fixed in the film structure. Therefore, when the a-Si photoreceptor is used within the apparatus of FIG. 1, ions, molecules or atoms present in an atmosphere produced by corona discharge or in any other atmosphere are easily adsorbed on the surface of the photoreceptor, and this reduces the electrical resistance of the photoreceptor on its surface, thereby increasing the chance of surface leakage of charges.
(2) In addition to the instability of electrical and photoconductive characteristics, the a-Si photoreceptor produces an indistinct image or white specks. In the first few cycles of copying operation, the photoreceptor has a surface potential profile after exposure as illustrated in FIG. 2(A). After a number of copying cycles, the surface potential has a profile as shown in FIG. 2(B) wherein the potential in the unexposed area is not sharply distinguished from the potential in the exposed area, and the potential in the unexposed area itself is lower than the value for the freshly used photoreceptor surface (the potential of which is depicted in FIG. 2(B) by the dashed line). This deteriorative change is most probably due to the surface leakage of charges discussed in (1) above.
(3) After its manufacture, the a-Si photoreceptor causes elimination of hydrogen atoms due to exposure to light or heat, and this is another source of the defects shown above.
(4) Because of the reasons mentioned above, the a-Si photoreceptor does not have sufficient long-term stability and this is another problem that requires early solution before the photoreceptor can be put to commercial use.
As a result of various studies to solve these problems, the present inventor has obtained the following observation. Before corona discharge, no ion, molecule or atom is adsorbed on the surface of the photoreceptor, which therefore, has the energy band structure shown in FIG. 3. After corona discharge, activated (or stable) ions are formed and adsorbed on the photoreceptor's surface, and at the same time, ambient ions are also adsorbed on the photoreceptor. This adsorption introduces impurity levels near the surface of the photoreceptor, causing a shift in the Fermi level (E.sub.F) in that area. Since the Fermi level near the surface of the photoreceptor must coincide with the Fermi level in the volume, the energy band for the surface area bends as shown in FIG. 4, and this band bending causes carriers to accumulate to form a space charge layer. The band bending occurs fairly deep into the photoreceptor and its depth (d) is sometimes up to 1 .mu.m below the surface of the photoreceptor. As a result of this accumulation of carriers due to band bending caused by adsorption of gases on the photoreceptor, the electrical conductivity of its surface may be appreciably increased to cause an indistinct image. FIG. 5 shows the electron state density profile of the photoreceptor on which gases have been adsorbed; in FIG. 5, E.sub.D is the donor level produced by the gas adsorption; E.sub.c shows the energy level of the bottom of the conduction band; E.sub.v shows the energy level of the top of the valence band.
An a-Si photoreceptor having an electron acceptor or donor substance adsorbed to its surface has been developed with a view to stabilizing the surface and controlling its electrical properties. However, being simply adsorbed onto the surface, the electron acceptor or donor substance is easily detached during cyclic use of the photoreceptor, and this substantially limits its service life. This problem becomes all the more significant because the electron acceptor or donor substance adsorbed on the photoreceptor is made of an organic compound which differs greatly in properties than the inorganic a-Si of which the bulk of the photoreceptor is made.